1. Field of the Invention
The present invention relates to a method of manufacturing a film for film capacitors and the film for the film capacitors.
2. Description of the Related Art
Capacitors can be classified into three kinds of capacitors, i.e., film capacitors, ceramic capacitors, and aluminum-electrolytic capacitors, according to kinds of dielectrics. Among these three kinds of capacitors, a film capacitor is superior to the other capacitors in terms of its characteristics that it is high in insulation property, is small in dielectric loss, and is small in characteristic change relative to a temperature or a frequency. This matter is described in Norifumi KANOU, “Technical Trend for Capacitor Films”, Convertech, No. 40, issue of July, 2006, and Denpa Shimbun, High Technology, issue No. 1142, issued on Jan. 24, 2008.
This film for film capacitors is practically available by ensuring that: a dielectric layer is formed of a resin such as a polypropylene resin (PP resin), a polystyrene resin (PS resin), a polyethylene terephthalate resin (PET resin), a polycarbonate resin (PC resin), a polyvinylidene fluoride resin (PVDF resin), polytetrafluoroethylene resin (PTFT resin), a polyimide resin (PI resin), a polyphenylene sulfide resin (PPS resin), or a polyethylene naphthalate resin (PEN resin); and that a metal vapor deposition layer is formed as an electrode on each surface of the dielectric layer.
A film for film capacitors, which is currently practically available, is a film obtained from four kinds of resins such as a PP resin, a PET resin, a PPS resin, and a PEN resin, and films obtained from other resins are hardly used, since there is a problem associated with cost efficiency or processing adaptability. This matter is described in Norifumi KANOU, “Technical Trend for Capacitor Films”, Convertech, No. 40, issue of July, 2006.
However, the film for film capacitors, made of the PP resin and the PET resin, show poor heat resistance, because the use temperature of the PP resin is 105 degrees centigrade or less and the use temperature of the PET resin is 125 degrees centigrade or less. Therefore, for example, if they are used as films for film capacitors of a hybrid vehicle which requires heat resistance of 150 degrees centigrade or more, it is inevitable to employ: (1) a method of installing large-sized cooling equipment without paying attention to a request for weight reduction; and (2) a method of installing a capacitor at, e.g., a driver's seat side which is distant from an engine room as a heat source without paying attention to space efficiency, and so problems associated with weight reduction and cost efficiency still remain unsolved.
In contrast to this, the film for film capacitors, made of the PPS resin, shows appropriate heat resistance, because its use temperature is 160 degrees centigrade or less. However, its applicability is limited, since it has low dielectric breakdown voltage and poor voltage proof. In addition, the film for film capacitors, made of the PEN resin, shows appropriate heat resistance because its use temperature is 160 degrees centigrade or less. However, its applicability is limited, since it has large dielectric loss and large temperature dependency of dielectric tangent. This matter is described in Norifumi KANOU, “Technical Trend for Capacitor Films”, Convertech, No. 40, issue of July, 2006, and Denpa Shimbun, High Technology, issue No. 1142, issued on Jan. 24, 2008.
In view of the foregoing description, as described in Japanese Patent Application Laid-open No. 2007-300126, a film made of a polyetherimide resin (PEI resin) is noted as a film for film capacitors. The film for film capacitors, made of a polyetherimide resin, is preferable as a film for film capacitors, because of its excellent heat resistance due to its high glass transition point of 200 degrees centigrade or more, and of its excellent voltage proof due to its high dielectric breakdown voltage, and of its small frequency dependency and temperature dependency of dielectric tangent.
A thin film whose film thickness is 10 microns or less is used as a film for film capacitors. However, the film made of the polyetherimide resin lacks in film lubricity (or slidability), and therefore, sometimes causes troubles such as interruption of winding operation or slitting operation of the film, appearance of wrinkles on the film, and sticking of the film to guide rolls, etc., during manufacturing operation of the film. In addition, if film blocking may occur and then the wound film is released during capacity assembling, the film may break and cause a problem in the assembling. Therefore, there is need to improve slidability in order to use the film made of the polyetherimide resin as a film for film capacitors.
Japanese Patent Application Laid-open No. 2007-300126 discloses the use of a film for film capacitors which is obtained by applying plasma treatment to a film surface made of a polyetherimide resin in a fluorination atmosphere to thereby form a fluorinated surface on the film surface made of the polyetherimide resin.
A film obtained by forming a fluorinated surface on a film surface made of a polyetherimide resin, described in Japanese Patent Application Laid-open No. 2007-300126, is capable of improving film slidability. However, as described in Japanese Patent Application Laid-open No. 2007-300126, in a case where the film surface made of the polyetherimide resin is formed by applying plasma treatment in a fluorination atmosphere, a plasma treatment device is needed, and there are problems of higher manufacturing cost as well as complicated manufacturing.
On the other hand, in order to impart slidability to the film for film capacitors, made of the polyetherimide resin, a method of adding a fluorine resin is considered to be effective for reasons that required slidability can be imparted without affecting voltage proof, and that the resin is thermally stable even in a molding processing temperature exceeding 300 degrees centigrade.
However, in a case where a film for film capacitors is molded by a melt extrusion molding technique using a resin composition obtained by adding a fluorine resin to a polyetherimide resin as a molding material, when the resin composition melted and kneaded in an extruder is extruded from a T-die, the polyetherimide resin and the fluorine resin are separated each other, and the polyetherimide resin unevenly adheres to a flow passage face of a lip portion of the T-die. This phenomenon will be described with reference to FIG. 5A and FIG. 5B. FIG. 5A and FIG. 5B are sectional views of the vicinity of the lip portion of the T-die of a film molded by a conventional film manufacturing method. That is, as shown in FIG. 5A and FIG. 5B, the polyetherimide resin has high affinity with metal, whereas the fluorine resin has peel property relative to metal. Accordingly, stripes are formed by a portion at which a polyetherimide resin P adheres to the flow passage face of a lip portion 7a and a portion at which a fluorine resin F exposes on the flow passage face and the polyetherimide resin does not adhere due to the peel property of the fluorine resin F, whereby streaks occur in the extruded film for film capacitors. A difference in stress generated between a streaked portion and a streak-free portion causes wrinkles and in turn, there occurs a problem that a value of the film for film capacitors is extremely degraded.
Further, since the processing temperature during the film melt extrusion molding exceeds 300 degrees centigrade, the polyetherimide resin P adhering to the flow passage face of the lip portion 7a oxidizes and deteriorates to form an eye mucus-like adherent, and as a result, die lines exerted by being frictionally scrubbed by the eye mucus-like adherent occur with the film for film capacitors.
The present invention has been made in view of the above-described circumstances, and it is an object of the present invention to provide: a method of manufacturing a film for film capacitors, which is capable of readily and reliably manufacturing films for film capacitors having excellent heat resistance, voltage proof, and slidability or the like in which occurrences of streaks, wrinkles, and die lines are restrained; and a film for film capacitors manufactured by this manufacturing method.